A clathrate hydrate or gas hydrate consists of a host molecule, which forms a hydrogen-bonded solid lattice, and a guest molecule (hereinafter, a guest gas) trapped inside the lattice. It refers to a crystalline compound wherein small molecules such as methane, ethane, carbon dioxides, etc., are physically trapped inside the three-dimensional lattice structure formed by hydrogen-bonded water molecules, without any chemical bonding.
The gas hydrate was first discovered in 1810 by Sir Humphry Davy of England. He reported during his Bakerian Lecture to the Royal Society of London that, when chlorine reacts with water, a compound resembling ice is formed, but the temperature thereof is higher than 0° C. Michael Faraday first found in 1823 that a gas hydrate is formed by a reaction of 10 water molecules with one chlorine molecule. Until now from then, the gas hydrate has been continuously studied as one of phase-change materials (PCMs). The main subjects of the study include phase equilibrium and formation/dissociation conditions, crystal structure, coexistence of different crystals, competitive compositional change in the cavity, etc. Besides, various detailed researches are being conducted in microscopic and macroscopic aspects.
At present, it is known that about 130 kinds of guest gases can be trapped in the gas hydrate. Examples include CH4, C2H6, C3H8, CO2, H2, SF6, etc. The crystal structure of the gas hydrate has a polyhedral cavity which is formed by hydrogen-bonded water molecules. Depending on the kind of the gas molecule and the condition of its formation, the crystal structure may vary to have a body-centered cubic structure I (sI), a diamond cubic structure II (sII) or a hexagonal structure H (sH). The sI and sII structures are determined by the size of the guest molecule and, in the sH structure, the size and the shape of the guest molecule are important factors.
The guest molecule of the gas hydrate naturally occurring in the deep sea and permafrost areas is mainly methane, and it has received attention as an environment-friendly clean energy source due to a small amount of carbon dioxide (CO2) emissions during combustion. Specifically, the gas hydrate may be used as an energy source to replace traditional fossil fuels and also for storage and transportation of solidified natural gas using the hydrate structure. Further, it may be used for separation and storage of CO2 to prevent global warming and may also be usefully used in seawater desalination apparatuses to dissociate gases or aqueous solutions.
Since the seawater provided as the host molecule contains various minerals including salts, a desalination process of separating the salts, etc., is necessary to use the components or to obtain fresh water for drinking. Several methods of desalinating the seawater have been presented and are practically employed in desalination facilities.
The most representative techniques of seawater desalination are evaporation method (thermal method) and reverse osmosis (RO) method. Because the evaporation method is a process of producing fresh water by evaporating and then condensing the seawater, it consumes a lot of energy and is uneconomical. The reverse osmosis method has been favored recently because it consumes less energy compared to the evaporation method. However, the biggest disadvantage of the reverse osmosis method is that the reverse osmosis membrane should be replaced periodically due to membrane fouling, thus increasing maintenance cost. Sufficient pretreatment is required for its resolution but when the seawater has a high salt concentration or contains many impurities the process maintenance cost will increase exponentially.
In addition to the above two methods, new methods for treating water based on the principle of forming gas hydrate, for example, aquatic resources concentration, drug separation, vitamin purification, wastewater treatment, water purification, brackish water desalination, seawater desalination, etc., have been developed. Of them, the seawater desalination process, for example, is characterized in that only pure water is used in the reaction for the formation of gas hydrate contaminants or salts contained in the seawater are excluded naturally. That is to say, the salts (e.g., NaCl) contained in the seawater are excluded when the gas hydrate is formed and the solid-state hydrate can be easily separated from the salt-rich filtrate. By dissociating the separated gas hydrate, pure water exclusive of the salts and contaminants can be produced.
The seawater desalination method based on the principle of gas hydrate formation allows the production of fresh water under mild conditions via a simple process. For example, when propane or fluoride gas is used as the hydrate medium, pure water can be separated under the condition of 5° C. and 5-10 atm. Since the system pressure of the seawater desalination method is lower than that of the reverse osmosis method requiring a pressure of 50-80 atm, and the method requires no additional cost for, e.g., replacement of membranes, fresh water can be produced at low cost. In order to separate the salt-excluded gas hydrate and the impurity-concentrated residual solution in the seawater desalination process using the gas hydrate method or a similar water treatment process, a dehydration process is essentially required. After the dehydration process, the hydrate crystals are compressed and packed. The core technology in the gas hydrate method is to efficiently separate/wash off the contaminants attached to the dehydrated and compressed solid-state hydrate crystals or the impurities between the crystals in order to obtain more purified water than that obtained by dissociation of the hydrate.
More specifically, the existing processes for seawater desalination disclosed in the references are as follows.
Both Korean Patent No. 10-0737183 and Korean Patent Application Publication No. 10-2009-0122811 provide a method or an apparatus for desalinating seawater using a gas hydrate. According to these patents, a gas hydrate is formed by injecting a single guest gas into a reaction chamber containing seawater. The gas hydrate is dehydrated and compressed to remove the impurities adsorbed on an outer surface of the gas hydrate. Finally, it is separated into fresh water and a gas by dissociation to thereby obtain fresh water. These methods are characterized in that, when the seawater and the guest gas are mixed in the reaction chamber, the formation of the hydrate is accelerated by spraying or using, e.g., a reaction promoter, while concurrently salts are dissociated from the seawater by separating the impurities during the dehydration process. However, although part of the impurities can be removed during the dehydration step, they lack the technical feature of efficiently removing the salts or contaminants attached to the surface of the gas hydrate crystals and between the crystals to obtain more purified water.
Meanwhile, International publication No. WO99/000330 (Jun. 17, 1998) [Marine Desalination Systems L.L.C. (US)] adopts a method of forming a gas hydrate by injecting a guest gas (methane) into a vertically long (a few hundred meters) column positioned vertically in a body of seawater and obtaining fresh water at a top portion of the column, and International publication No. WO07/145740 (May 8, 2007) [Marine Desalination Systems L.L.C. (US)] aims at improving productivity by continuously performing hydrate formation and dissociation using a HART module.
However, although these patents aim at improving the efficiency of gas hydrate formation in manufacturing gas hydrate, their efficiencies of salt removal are not substantially high. Although it is necessary to efficiently remove the impurities attached to the surface of the gas hydrate crystals and between the crystals during the hydrate formation process as described above, these patents lack such a technical feature. In addition, most patents relating to seawater desalination or water treatment using a gas hydrate merely mention a guest gas without specifying the gas. If only one guest gas is used, dissociation occurs in short time when pure water is obtained by dissociating the gas hydrate. As a result, the impurities present on the surface of the gas hydrate crystals or between the crystals cannot be efficiently removed and there is a limitation in obtaining pure water.
As described above, although techniques that allow for water treatment based on the principle of gas hydrate are being developed, they merely accelerate the formation of the gas hydrate using an ultrasonic apparatus, a reaction promoter, etc., and are limited in efficiently removing impurities such as salts.
(Patent document 1) Korean Patent No. 10-0737183 (Jul. 10, 2007) (Dowell Technology Co., Ltd.).
(Patent document 2) Korean Patent Application Publication No. 10-2009-0122811 (Dec. 1, 2009) (Korean Institute of Industrial Technology).
(Patent document 3) International publication WO99/000330 (Jun. 17, 1998) [Marine Desalination Systems L.L.C. (US)].
(Patent document 4) International publication WO07/145740 (May 8, 2007) [Marine Desalination Systems L.L.C. (US)].